Carbon fibre brush



Aug. 18, 1970 L, ETAL CARBON FIBRE BRUSH Filed Feb. 29, 1968 QIQQ UnitedStates Patent Oflice 3,525,006 Patented Aug. 18, 1970 3,525,006 CARBONFIBRE BRUSH Norman Lawrence Parr, Wareham, Dorset, Sam Bolshaw,

Bath, Somerset, and Edwin Richard Wells May, Moordown, Bournemouth,England, assignors to National Research Development Corporation, London,England, a British corporation Filed Feb. 29, 1968, Ser. No. 709,444Int. Cl. H011 39/26 U.S. Cl. 310251 4 Claims ABSTRACT OF THE DISCLOSUREA carbon brush comprising an aligned array of juxtaposed carbon orgraphite fibres wherein the contact surface of said brush is formed bythe ends of said fibres. The fibres may be bonded to each other for apart of their length either by means of an epoxy resin or byelectroplating the ends of said fibres.

This invention relates to electrical brushes having an improved currentcarrying capacity by means of more efiicient surface contact.

According to the present invention a carbon brush comprises an alignedarray of juxtaposed carbon or graphite fibres wherein the contactsurface of the brush is formed by the ends of the fibres.

Referring, by way of example, to the accompanying drawing, whichillustrates a brush according to the present invention, an aligned arrayof juxtaposed carbon or graphite fibres 1, which are bonded to eachother over a portion of their length 2 by an epoxide resin, areencapsulated in a soft metal can 3 open at one end. Electricalconductivity at the non-contact end of the brush is ensured byelectro-plating the ends 4 of the fibres which in turn are soldered tothe closed end of the can 5.

Carbon brushes of dimensions /2" x 1" x 2 long have been produced inaccordance with the invention. Sufficient carbon or graphite fibres ofsuitable length and diameter are placed in a three sided metal containerand compressed with the fibres protruding from the container by about atone end and about 4 at the other. The fourth side of the container isthen crimped and soldered into position whilst the fibres are undercompression. The assembly is then placed vertically in a vesselcontaining diluted epoxy resin (e.g. Araldite MY 753 (registeredtrademark) diluted with %-10% acetone) in such a manner that resin isdrawn, by capillary attraction, into the fibres to a predeterminedheight. The whole is then allowed to stand" until the resin cures.Excess resin is then removed and the resin-bond end of the fibre blockfaced level with the container to expose the ends of the fibres, whichare electroplated to a continuous sheet of copper and then tinned with60/40 solder. The end closure of the container, which has beenpreviously tinned, is then folded down to meet the tinned end of thefibre block and the whole sweated together. A flexible electricalconnection is then soldered to the container as desired. A suitablematerial for the container or can is copper sheet of 0.02 in gauge forthis size of brush.

Experience has shown that resistance heating in individual fibres canlead to overheating of the brush and thus produce degradation of theepoxy resin. It has been found that this can be overcome by usingshortened brushes and by bonding the fibres at the non-contaot end ofthe brush by low energy copper electro-plating of the tips of the fibresand thereby dispensing with the need to anchor the fibres with epoxyresin.

'In one example, small brushes in length by A" x 1" have been madewithout the use of epoxy resin. The fibres are encapsulated in a coppercontainer as previously described and the assembly is then placedvertically in a copper plating bath. Electrolyte is drawn into thebundle of fibres by capillary attraction and the ends of the fibres areplated to a depth of .005. Further time in the plating bath results inthe encasement of the ends of the fibres in a continuous matrix ofcopper, which is then tinned with /40 solder and the end closure of thecontainer, which has been previously tinned, is then folded down to meetthe tinned end of the fibre block and the whole sweated together.

It has also been found that control of the build-up of resistive oxidefilms on the commutator can be accomplished by the introduction of aselected number of fibres in the brush (e.g. glass, metals or otherabrasive materials) to impart a limited degree of wear.

We claim:

1. A carbon brush comprising an aligned array of juxtaposed carbonfibres wherein the contact surface of said brush is formed by the endsof said fibres said array including a number of fibres of abrasivematerial.

2. A carbon brush according to claim 1 wherein said abrasive fibres areof glass.

3. A carbon brush comprising an aligned array of juxtaposed carbonfibres wherein the contact surface of said brush is formed by the endsof said fibres said fibres being bonded to each other for a part oftheir length by an epoxy resin.

4. A carbon brush according to claim 3 wherein said array includes anumber of fibres of abrasive material.

References Cited UNITED STATES PATENTS 539,453 5/1895 Thomson 310--248539,454 5/1895 Thomson 310-248 3,382,387 5/1968 Marshall 310-248 MILTONO. HIRSHFIELD, Primary Examiner L. L. SMITH, Assistant Examiner U.S. C1.X.R. 310252

